This example is not a real world example, in a business application there are many of these scenarios which would add up. If you can save even half of second on an interaction it's worth it. And last, if you become aware of other ways to solve a problem faster why not
the why not is easy, cpu time is cheap, developer time isn't. if the optimizations are bad for the readability of the source code and imperceptible to end users, you're taking on bad tech debt. the reverse of that is true though too, don't get me wrong. "good" tech debt would mean taking on slightly more obscure code so that you can bring noticeably faster response times directly to the user.
for the real world use, yeah, you definitely can find a lot of inefficiencies that add up to a poorly performing app, but you have a pretty generous time budget before users are going to disengage with the application. if you're doing something like the example (simple sorting, filtering, restructuring on large datasets), you generally have anywhere from 1,000ms to 10,000ms before a user is going to think there's a serious problem here.
in other words you'd have to 200 plus bad chains together before you finally reached a point where users are noticing and reacting negatively.
there's also the case we haven't even considered where usually when someone is filtering then mapping it's because filtering is fast and cheap, while a much heavier lift is in the map. in those kinds of scenarios your advice would actually end up costing more time—both on the wall and on the cpu.
I think we're discussing the same thing from two different angle, but let's just say if developers take a bit of time upskilling and finding new ways to solve problems more efficient you'll have a performing app and save time finding where to optimise in a large code base.
Back to your point, a developer spending a bit of time reading will help much more than simply code and wait for a bug or a ticket to be raised later on performance.
Last you're just taking one of the points and generalising it, perf tuning is a broad aread, this is one of the thousands
and in those cases i would rather a past developer had put their time and energy into making sure their intent is clear before getting clever.
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